The invention relates generally to fiber optic sensing technologies. In particular, fiber optic sensors are used to detect conditions within a well.
Available electronic sensors measure a variety of values, such as, pH, color, temperature, or pressure, to name a few. For systems that require a string of electronic sensors over a long distance, e.g., twenty to thirty kilometers or longer, powering the electronic sensors becomes difficult. Conventionally, the powering of electronic sensors requires running electrical wire from a power source to each of the electronic sensors. Powering electronic sensors electrically has been a problem in the petroleum and gas industry. However, electric wires spanning such long distances create too much interference and noise, thereby reducing the accuracy of the electronic sensors.
Optical fibers have become the communication medium of choice for long distance communication due to their excellent light transmission characteristics over long distances and the ability to fabricate such fibers in lengths of many kilometers. Further, the light being transmitted can also power the sensors, thus obviating the need for lengthy electrical wires. This is particularly important in the petroleum and gas industry, where strings of electronic sensors are used in wells to monitor down hole conditions.
As a result, in the petroleum and gas industry, passive fiber optic sensors are used to obtain various down hole measurements, such as pressure or temperature. A string of optical fibers within a fiber optic system is used to communicate information from wells being drilled, as well as from completed wells. For example, a series of weakly reflecting fiber Bragg gratings (FBGs) may be written into a length of optical fiber, such as by photoetching. As is known in the art, the distribution of light wavelengths reflected from an FBG is influenced by the temperature and strain state of the device to which the FBG is attached. An optical signal is sent down the fiber, which is reflected back to a receiver and analyzed to characterize the length of optical fiber. Using this information, down hole measurements may be obtained.
Many methods are utilized to characterize these sensor-containing lengths of optical fiber, including but not limited to optical reflectometry in time, coherence, and frequency domains. Due to spatial resolution considerations, optical frequency-domain reflectometry (OFDR), is a technique under investigation for use in oil well applications. OFDR is capable of spatial resolution on the order of 100 microns.
In OFDR, the probe signal is a continuous frequency modulated optical wave, such as from a tunable laser. The probe signal, which is optimally highly coherent, is swept around a central frequency. The probe signal is split and sent down two separate optical paths. The first path is relatively short and terminates in a reference reflector at a known location. The second path is the length of optical fiber containing the sensors. The reference reflector and the sensors in the length of optical fiber reflect optical signals back toward the source of the signal. These optical signals are converted to electrical signals by a photodetector. The signal from the reference reflector travels a shorter path, and a probe signal generated at a particular frequency at a single point in time is detected at different times from the reference reflector and the sensors. As such, at any point in time, the signal at the receiver is a signal from the reference reflector and a signal from the sensors at slightly different frequencies due to the sweeping nature of the tunable laser source. A difference frequency component stemming from the time delay in receiving the signal from the reference reflector and the sensors in the optical fiber can be observed in the detector signal. The frequency of the difference frequency component determines the position of the sensor on the fiber and the amplitude is proportional to the local back scattering coefficient and optical power. Performing a Fourier transform of the detector signal, one can simultaneously observe the back scattered waves from all points along the fiber under test.
The operational properties of an OFDR are governed by the wavenumber spacing, v, the wavelength sweep range, R, the data acquisition frequency, f, and the sensing fiber length, LS. As discussed in greater detail herein, the sensing length LS is a simple function of L, but L also affects v, R, and f. Increasing the sensing length LS by making L arbitrarily large consequently reduces the wavelength sweep range R and increases the data acquisition frequency f to impractical values. Similarly, the wavelength sweep range R can be restored by increasing N, but this increase comes at the expense of the size of the data set required and the amount of time required for FFT computation. Maximizing the sensing length while maintaining speed and efficiency is a difficult challenge in the successful construction of an OFDR. Given these challenges, a typical OFDR system is currently limited to a sensing length of about 100 meters. Therefore, a need exists in the art for efficiently extending the useful sensing lengths for OFDR systems.